His great work, entitled Template:Lang (Rhōmaikē archaiologia, Roman Antiquities), embraced the history of Rome from the mythical period to the beginning of the First Punic War. It was divided into twenty books, of which the first nine remain entire, the tenth and eleventh are nearly complete, and the remaining books exist in fragments in the excerpts of Constantine Porphyrogenitus and an epitome discovered by Angelo Mai in a Milan manuscript. The first three books of Appian, and Plutarch's Life of Camillus also embody much of Dionysius.

His chief object was to reconcile the Greeks to the rule of Rome, by dilating upon the good qualities of their conquerors and also by arguing, using more ancient sources, that the Romans were genuine descendants of the older Greeks. According to him, history is philosophy teaching by examples, and this idea he has carried out from the point of view of the Greek rhetorician. But he carefully consulted the best authorities, and his work and that of Livy are the only connected and detailed extant accounts of early Roman history.